LAUGHTON
ELECTRONICS

a system of Safety
Interlocks
is retrofitted to existing equipment

Increasingly
strict safety standards are an issue of concern to employers.
Government inspections make the matter impossible to ignore. This
article describes how a valuable piece of equipment was upgraded to
meet modern requirements.

Kugler with a panel open.*
Click
to expand any image. *

The trunk lines run
along the perimeter, linking the
six junction boxes to the main electrical panel.

Closeup of a switch and actuator

Closeup of a junction box. One of the six setup
LEDs is illuminated, visible through the tough,
translucent plastic enclosure.

The junction boxes and controller use my analog multiplexing scheme to
communicate.

The G9SX and (upper right) the controller.

*
Click
to expand any image. *

This book-binding
machine,
made in Germany about 20 years ago by Kugler Automation, is central to
one of my clients' operation. Therefore it was a
serious matter when the Ontario Ministry of Labour gave notice that
there was
a problem with safety. The machine needed to be upgraded or replaced
— or else
it would be shut down and locked out. Replacing the machine was out
of the question, so I was asked to plan and install a system
of safety interlocks.

The machine is about 10 meters long (over thirty feet) and has a
series of removable panels close to floor level. Behind the panels are
the shafts, gears and cams that drive the various reciprocating
components at each station. Obviously these mechanisms are
potentially dangerous. My job was to arrange for
the machine
to stop automatically if any of the panels was opened. This
"simple" concept entailed some detailed considerations. These include:

switch
selection
Ordinary switches are inappropriate.

cable
harnesses

The added wiring must be compact.

a
human
interface
The system must be able to indicate which of the 29 switches has
triggered an open-panel shutdown.

Switch Selection

Because a tamper resistant switch was required I
selected Omron's type D40A for the project. This switch is
two-piece "non-contact" type, with a solid-state sensor that mounts
on the chassis
and a magnetic actuator that can attach to a door or panel.
The sensing range (sensor to actuator) can be as much as 1 cm,
which helps to overcome any minor mis-alignment that may exist with the
Kugler
panels. The panels hang, somewhat sloppily, on hooks (as shown
in the photo).
The aluminum parts behind the actuator serve to locate it adjacent to
the sensor.
Also, the sensor itself mounts on a 1/8 inch aluminum shim. This
reduces proximity to
the machine's steel frame — a ferrous object which might
otherwise interfere magnetically,
reducing sensing range and increasing the likelihood of a false alarm.

Cable Harnesses and
signal multiplexing

Adding extra cabling to this machine was problematic.
The wiring conduits were already stuffed close to
capacity, and in any case it would be absurd to try to run 29
individual
switch wires back to the electrical panel. The
situation
required a series of secondary nodes where individual
switch wires could connect to a trunk line. Six
"junction
boxes" are the nodes that serve this function. Each junction box has a
maximum
of six switches connected to it.

It was important that the
trunk cable itself be minimized. For example, it
would be undesirable to run a fat, 29-conductor cable
through the machine — due
to space limitations, and also because that would entail 29-pin
disconnect plugs. (Disconnect plugs allow
the machine to be partially disassembled in the event of its being
moved.) Instead, the plan uses a pair of
much thinner cables: only three conductors, plus a few D40A
specific signals. These cables run along the outer and inner perimeter
of the
machine, as show in the diagram (above left).

The reduced
conductor count in the trunk cables is
achieved by signal multiplexing. Specifically, each
junction box requires just a single conductor to transmit the state of
all six switches attached to it.
Nevertheless, the circuitry in each junction box is
delightfully simple, thanks to an original analog encoding
scheme. Each switch is associated with a unique voltage, determined
by a zener diode that connects it to the analog bus for that box.
Multiple switches active on the same bus do not mutually interfere
because all the zeners are individually current-limited to a known
value (3.5 mA, which also conveniently powers each switch's setup LED).
The controller, located
in the main electrical panel, tests voltage and
current
in order to discriminate between signals on the analog buses.

The remaining four
conductors in the trunk cables are for 24V power and for a signal
loop used by the D40As. The loop is managed by an Omron G9SX (a D40A
accessory). It's capable of detecting wiring faults in the loop, such
as tampering
or a short to +24 — a hazard which might prevent recognition
of an open- panel signal.

The G9SX, located in the
main electrical panel, is what
triggers the actual machine stop circuit. Its output is just a
simple Go / No-Go decision. The multiplexed signals operate
independ­ently, using the Auxiliary output on each switch. This
is
what identifies individual
switches in order to support
the Human Interface (signal system).

Human Interface (signal
system)

The project includes a large, conspicuously-mounted LED lamp
on a pole. Its function is to inform staff which switch has been
triggered.
Although it's usually obvious when a panel is open, the signal
acts to identify invisible real-world glitches such as a switch or
actuator that
has gotten knocked out of alignment, yielding a spurious machine stop.
Thus it is a means of minimizing machine down
time — always critical in a production environment.

To indicate which switch
has been triggered, the LED lamp blinks out the necessary
information. For example, 2
flashes followed by 6
flashes would
indicate that switch 26
has been triggered. The
system
can manage any combination of panels open simultaneously.
Multiple panels open simply result in multiple flash codes in
succession.

Alternative or additional
signalling can easily be arranged since an industry-standard 24V PLC
interface is provided. One PLC output
and two PLC inputs are required (clock, data & strobe,
respectively). But
hookup to an external PLC is strictly optional. Typically
the system is self-contained, relying on the microprocessor inside the
controller and driving the LED lamp directly.

Conclusion

This
is a project that leverages the client's capital investment by
extending the useful lifetime of the machine.
Assemblies,
code and full documentation are available.